Diseases of the nervous system are particularly apt to affect the offspring, and often the inherited condition repeats that of the parents.  This is due to the fact that most of the nervous diseases depend both upon intrinsic factors which consist in some defective condition of the nervous system representing a variation, and extrinsic factors due to environment or occupation which make the basal condition operative.  The definite relation between alcoholism and insanity is due to alcohol acting not as an intrinsic but an extrinsic factor, bringing into effectiveness the hereditary weakness of the nervous system.  The influence of heredity in producing insanity is variously estimated at from twenty-six per cent to sixty per cent of all cases.  This great difference in the estimation of the hereditary influence is due to the personal equation of the statistician, and the care with which other factors are eliminated.  In the more severe form of the hereditary degeneration the same pathological conditions are repeated in the descendants.  In certain cases the severity of the condition increases from generation to generation.  According to Morel there may be merely what is recognized as a nervous temperament often associated with moral depravity and various excesses in the first generation; in the second, severe neuroses, a tendency to apoplexy and alcoholism; in the third, psychic disturbances, suicidal tendencies and intellectual incapacity; and in the fourth, congenital idiocy, malformations and arrests of development.  There are some very definite data with regard to inheritance in the nervous disease known as epilepsy.  The essential condition in this consists in attacks of unconsciousness, usually accompanied by a discharge of nerve force shown in convulsions, the attack being often preceded by peculiar sensations of some sort known as the aura.  In the most marked forms of the affection heredity plays but little part, owing to the early supervention of imbecility and helplessness, and it is a greater factor in the better classes of society than in the proletariat.  In the better classes, owing to the greater care of the cases and the avoidance of exciting causes of the attacks, the disease is better controlled and rarely advances to the extent that it does among the poor.  The association of epilepsy and alcoholism is especially dangerous, for a slight amount of alcohol may greatly accentuate the disease.  In five hundred and thirty-five children in whose parentage there were sixty-two male and seventy-four female epileptics, twenty-two were born dead, one hundred and ninety-five died from convulsions in infancy, twenty-seven died in infancy from other causes, seventy-eight were epileptics, eleven were insane, thirty-nine were paralyzed, forty-five were hysterical, six had St. Vitus’s dance, one hundred and five were ordinarily healthy.  That variations in the nervous system which produce more or less unusual mental peculiarities and which do not take the form of nervous disease are inherited, the most superficial consideration shows.  A child in its mental characteristics is said to take after one or the other of its parents, certain habits and mental traits are the same, often even the handwriting of a child resembles that of a parent.